Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 160
Address Resolution

The purpose of address resolution is to determine the link-level address of a destination given only its IP address. This is performed only for those IP addresses that are local (hop count set to 1 for these messages). When a multicastable interface starts, it must join both the all-nodes multicast group and the solicited-node multicast group. This enables the node to receive and process packets without having all of its addressing established. In fact, a node must keep the multicast addresses until all addressing has been resolved. Address resolution consists of sending a Neighbor Solicitation message and waiting for a Neighbor Advertisement using multicast addressing. The solicitation is sent to the solicited-node multicast address corresponding to the target address. The solicited-node multicast address is a link-local scope multicast address that is computed as a function of the solicited target’s address. The solicited-node multicast address is formed by taking the low-order 32 bits of the target IP address and appending those bits to the 96-bit prefix FF02:0:0:0:0:1 to produce a multicast address within the range of FF02::1:0:0 to FF02:: 1:FFFF:FFFF. For example, the solicited node multicast address corresponding to the IP address 4037::01:800:200E:8C6C is FF02::1: 200E:8C6C. IP addresses that differ only in the high-order bits (e.g., due to multiple high-order prefixes associated with different providers) will map to the same solicited-node address, thereby reducing the number of multicast addresses a node must join. In response to this request (the sender may send it multiple times if no response is found within a certain period of time), a Neighbor Advertisement should be generated by the remote node. The originating node should receive this packet and update its Neighbor cache with the information in the received Neighbor Advertisement (the link-layer information). The MAC address is set as previously indicated by taking the low-order 32 bits of the target IPv6 address and prepending 3333 to that address, which is the IPv6 all-nodes MAC multicast address.

One more check is accomplished each time a Neighbor cache (link-layer information) entry is accessed while transmitting a unicast packet: The sender checks Neighbor Unreach-ability Detection-related information according to the Neighbor Unreachability Detection algorithm. This is not so much a protocol as keeping an eye on the progression of the upper-layer protocols with this address. This unreachability check might result in the sender transmitting a unicast Neighbor Solicitation to verify that the neighbor is still reachable.

If at some point communication ceases to proceed, as determined by the Neighbor Unreachability Detection algorithm, next-hop determination may need to be performed again. For example, traffic through a failed router should be switched to a working router. Likewise, it may be possible to reroute traffic destined for a mobile node to a “mobility agent.”

Note that when a node redoes next-hop determination, there is no need to discard the complete Destination cache entry. In fact, it is generally beneficial to retain such cached information as the PMTU and round-trip timer values that may also be kept in the Destination cache entry.

Next-hop determination is done the first time traffic is sent to a destination. As long as subsequent communication to that destination proceeds successfully, the Destination cache entry continues to be used. All of this is detailed further later in the book.